As for ceramic honeycomb structures which are recently used as catalyst carriers for purification of exhaust gases and others, attempts have been earnestly made to reduce the heat capacity of cell walls supporting the catalysts by reducing the thickness of the cell walls for rapidly raising the catalyst temperature at the time of starting of engine to improve purification performance. Specifically, at present, a thickness of the cell walls of 0.1–0.2 mm is mainly employed, and even a thicknes of less than 0.1 mm is employed.
Furthermore, the attempt to thin the cell walls is also made for large-sized honeycomb structures, and at present, large-sized products of more than 150 mm in outer diameter which are thinned in walls are put to practical use.
On the other hand, conventional apparatuses for producing honeycomb structures and method for producing them are generally those according to which honeycomb structures extruded from an extruder are continuously extruded in a direction perpendicular to the gravity direction while placing them on a plurality of cradles having concave faces corresponding to the shape of the outer peripheral side face (JP-B-64-6916, etc.).
As an automatic cutting device suitable for the apparatuses for producing honeycomb structures and method for producing them by the continuous extrusion molding, there is disclosed an automatic cutting device having a cradle on which the honeycomb structures are placed, a carrying path for moving the cradle with the honeycomb structures placed thereon, a speed sensor for sensing the extrusion speed of the honeycomb structures, a cutting device carrying out the cutting of the honeycomb structures while moving in the moving direction of the honeycomb structures and at the same speed as the extrusion speed of the honeycomb structures which is sensed by the speed sensor (the same patent publications referred to as above).
However, since according to the conventional apparatuses and methods, the honeycomb structure is extruded in the direction perpendicular to the gravity direction, there is a problem that its own weight is apt to be applied in the thickness direction of cell wall which is structually small in strength. Therefore, in case a honeycomb structure which is considerably reduced in strength due to the reduction in wall thickness or a honeycomb structure in which its own weight is apt to be applied in the thickness direction of the cell wall due to increase in size is produced, there occurs distortion of outer wall such as rupture, or distortion of cell wall such as cell twisting or mesh creasing owing to its own weight, and these are severe causes to hinder reduction in thickness of walls and increase in size of the honeycomb structure.
As a result of intensive investigations conducted by the inventors in an attempt to solve the above problems, first, it has been found to employ the gravity direction or an oblique direction at an angle of smaller than 30° with the gravity direction (hereinafter referred to as “gravity direction, etc.” as the extrusion direction of the extruding machine so as to apply the own weight of the honeycomb structure mainly in the lengthwise direction of the cell wall which has structurally the highest strength.
However, it has been found that according to the apparatuses having the above extrusion direction, there is a new problem that distortion is apt to occur in the extruded honeycobm structure due to the force in the diameter direction with extension in length of the extruded honeycomb structure, being different from the conventional apparatuses in which cradles are successively supplied to the outer peripheral side surface.
That is, when the extrusion direction is the gravity direction, etc., it has been found that the honeycomb structure extruded becomes difficult to maintain its inherent attitude even by a very small force in the diameter direction with extension of the length, and there occurs distortion of outer wall due to bend or rupture of the honeycomb structure or distortion of cell walls such as cell twisting at the position of opening of die at which forces in the diameter direction are apt to be concentrated owing to the factors such as cutting stress caused by cutting with a small-gage wire, vibration of machine and others.